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I had a quick query regarding the setup of a reaction. The procedure that I am working from states that I need to put my reaction mixture under reflux for 3 hours. The solvents used are diethyl ether (30 mL) and THF (10 mL). How do you determine the temperature needed? Do I go via the boiling point of diethyl ether or THF?

If more details are needed regarding the reaction I am carrying out then please let me know. But my query leans more towards the determination of mixed solvent systems in general and how one would determine the reflux temperature needed.

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    $\begingroup$ Why do you need to know an exact number?: you can turn down the heat when reflux is achieved and that will be the right temperature. This gives a simple self-calibrating way to get the right temperature where you don't even need to measure the temperature. $\endgroup$ – matt_black Sep 10 '20 at 19:39
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    $\begingroup$ Put a thermometer in your reaction when it is refluxing, and you will know. $\endgroup$ – Karl Sep 10 '20 at 19:39
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    $\begingroup$ Or do you mean the bath temperature outside your reaction vessel? $\endgroup$ – Karl Sep 10 '20 at 19:50
  • $\begingroup$ Hi Matt and Karl, I am a student and didn't know this, I'm very new to some of the practical chemistry I'm carrying out. The paper I was working from didn't give the temperature used so I just wondered. Anyway thanks for your help $\endgroup$ – Jamie Sep 10 '20 at 19:58
  • $\begingroup$ You don't have to know the temperature if you supposed to reflux a reaction mixture. You add correct amounts of reactants and solvent(s), and heat it until mixture is boiling. However, make sure you have efficient cooling system for your upright condenser, specifically when you are heating diethyl ether mixtures (no flame; use a heating mantle or a hot plate). If you really want to know the temperature for reporting purposes, follow Karl's instructions. $\endgroup$ – Mathew Mahindaratne Sep 10 '20 at 20:55
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Trial and error, but most often the boiling point of the lower-boiling solvent.

The absolutely correct answer would be to look at a phase diagram, determine whether you have a positive or negative azeotope (or non at all), find out where on the phase diagram you are and then derive the temperature. That would tell you approximately at what temperature your mixture will boil – after you factored in boiling point increase due to dissolved compounds.

In practice, you need to set your oil bath to some 5 or 10 degrees higher than the boiling point of the lower-boiling solvent anyway. So you can just ballpark it, selecting a temperature that would keep the lower-boiling solvent refluxing. Come back after 15 minutes, if it is not refluxing, increase by 5 or 10 degrees.

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  • $\begingroup$ And for your specific case: refluxing ether is room temperature ;) $\endgroup$ – Jan Sep 17 '20 at 7:45

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